1. Field of the Invention
The present invention relates to a technique of using a plasma generator to purify exhaust gas discharged from an internal combustion engine.
2. Description of the Related Art
There is proposed a method of using plasma for an exhaust gas purifying technique of a lean burn engine (mainly Diesel engine). The method of using the plasma is a technique in which electromagnetic energy is imparted to the exhaust gas by discharge to put the exhaust gas into a plasma state, and thereby decomposition reaction of toxic substances is promoted to generate a direct purification reaction or a purification reaction with a catalyst or the like (for example, see Japanese Patent Publication Laid-Open No. H6-10652).
Usually a method in which the alternating current discharge is utilized to generate the plasma, and there are proposed many techniques of controlling the plasma conditions improve purification efficiency (for example, see Japanese Patent Publication Laid-Open Nos. H5-59934 and 2001-46910).
Japanese Patent Publication Laid-Open No. H5-59934 discloses a configuration in which corona discharge is applied to the exhaust gas of the internal combustion engine to perform denitration. In the configuration, discharge voltage to an electrode which performs the corona discharge is controlled according to an operation condition of the internal combustion engine.
Japanese Patent Publication Laid-Open No. 2001-46910 discloses a technique of controlling an interval of the intermittent discharge corresponding to an exhaust gas flow rate in a configuration in which high alternating-current voltage is applied to the internal combustion engine exhaust gas to perform the intermittent discharge. These techniques are aimed at the improvement of exhaust gas purification efficiency and reduction of electric power consumption.
However, in the conventional techniques, there is a limitation to pursuance of the purification efficiency without largely increasing the electric power consumption. That is, the pursuance of the purification efficiency inevitably leads to the increase in electric power consumption.